Depending upon the operational environment of a particular fuel system, certain requirements are imposed on the design thereof for ensuring, inter alia, that fuel is supplied continuously and at a predetermined rate, particulate matter such as ice or debris in the fuel supply is filtered prior to ingestion by the engine, and the spillage of fuel is minimized in an emergency situation. For example, it will be appreciated that fuel systems for aircraft must ensure continuous operation of the fuel system in icing conditions and in the presence of debris in the fuel system. Furthermore, such fuel systems must provide means for mitigating the hazards of fuel spillage in the event of a fuel line rupture, i.e., in a crash or as a result of a ballistic impact.
More specifically, with respect to ice in the fuel supply/supply lines, the Federal Aviation Administration (FAA) requires that aircraft fuel systems operate in the presence of free water in the fuel supply which, at sub-freezing temperatures, can precipitate out of the fuel and form ice on system components, e.g., inlets, pumps, etc. As such, ice accretion can restrict fuel flow and result in engine "flame-out". Conventionally, this requirement has been addressed by the use of fuel additives, such as ethylene glycol monomethyl ether (also known as Prist.RTM. fuel additive produced by PPG Industries, Chemicals Group, located in Pittsburgh Pa.), to abate the formation of ice on system components. While such additives are effective for their intended purpose, the toxicity of such additives is increasingly raising concerns regarding safety, i.e., it has been reported that long-term exposure to such additives may cause health hazards.
With respect to debris or foreign objects which may enter the fuel supply, the FAA also requires that debris be filtered so as to avoid blockage of fuel supply lines or engine flow paths. One prior art system for satisfying this requirement employs filtering apparatus, i.e., screens or strainers, situated downstream of a boost pump disposed internally of the fuel tank. This system, which is a pressurized fuel system, provides a primary flow path through the filter apparatus and a secondary flow path which circumvents the filter apparatus in the event that fuel flow therethrough becomes restricted. More specifically, the secondary flow path includes spring-loaded check valves which open in response to a threshold level of back pressure in the fuel system, thereby facilitating a bypass flow of fuel to the engine.
Disadvantages of pressurized fuel systems relate to the weight and complexity associated with the internal boost pump, e.g., the need to route electrical power into the fuel tank, and the additional weight of bypass fuel lines and check valves. Another disadvantage of pressurized fuel systems relates to the increased potential for fuel discharge in the event of a fuel line rupture. The shortcomings of pressure fuel systems in this regard are discussed below.
With respect to requirements to minimize the discharge of fuel should a crash or other event fracture the fuel line, it is common practice to employ self-sealing break-away valves at various locations in the fuel system. These valves are designed to fracture or break before the failure of other components in the fuel system and immediately seal upon fracture to minimize fuel spillage. Despite this safety feature, ballistic damage or the dynamics of a crash may fracture or sever the fuel line without activating the break-away valves. In such event, pressure fuel systems can continue to pump fuel out of the fuel tank and produce or further aggravate a hazardous condition.
To further enhance system safety, alternative fuel systems have been developed which utilize suction pumps, i.e., rather than boost pumps, to deliver fuel to the engine. The suction pumps are engine driven and disposed externally of the fuel tank. As such, in the event of a fuel line rupture, air is drawn into the fuel system, thereby producing a benign failure mode. While suction fuel systems provide significant advantages in this regard, such systems are highly sensitive to pressure drops in the fuel system. That is, insofar as the maximum pressure differential produced therein is limited by several factors including, the lift and vapor handling capability of the suction pumps, suction fuel systems cannot employ components which produce large pressure drops. For example, suction fuel systems cannot utilize pressure actuated check valves, typically used in pressure systems, to circumvent flow around a filtering apparatus. Furthermore, the prior art suction fuel systems cannot tolerate large pressure drops due to an accumulation of ice on fuel system components. Accordingly, such prior art systems must resort to the use of anti-icing additives and the inherent disadvantages associated therewith.